gh-104683: Modernise Argument Clinic parameter state machine

[erlend-aasland]

Use enum and pattern matching to make the code more readable.

• Issue: Modernise code in Tools/clinic/ #104683

[AlexWaygood]

Great idea; this is a big improvement in my opinion. A few small comments, mostly about idiomatic usage of enums:

[AlexWaygood]

(I take it you've checked that all this code is well covered by our tests? :)

[erlend-aasland]

(I take it you've checked that all this code is well covered by our tests? :)

It's pretty good checked; I ran into test failures bco. some hickups during implementation :) However, I'll do a proper coverage check before landing.

[AlexWaygood]

Looks great, providing the test coverage is good. I left a comment about a possible enhancement you could do with the enum, but I think it's probably a case of YAGNI.